This thesis describes experiments which were carried out to fabricate new silicon nitride-based ceramic composites and to analyse their microstructural and physical characteristics. Possible applications considered were those involving high hardness or wear-resistant properties. Two composite systems were investigated which had either silicon carbide platelets or cubic boron nitride particles, dispersed in a silicon nitride matrix (or binder) phase. The composites were fabricated using unconventional techniques, involving the hot-pressing of hybrid, laminated composites and ultra-high pressure sintering (at pressures exceeding 4 GPa). Each of these techniques was considered to offer a means of improving the fracture toughness of the ceramics, in addition to the high hardness. Hybrid, laminated SiC-Si3N4 composites were succesfully fabricated by hotpressing composites consisting of structurally arranged tape cast laminae with different proportions of SiC platelets. These composites showed promising indications of enhanced fracture toughness through delamination along laminar interfaces in addition to microstructural toughening mechanisms within the laminae. Fabrication by ultra-high pressure sintering was found to be detrimental to the mechanical properties of the SiC-Si3N4 composites and it is suggested that the microstructural toughening effects due to the SiC platelets were rendered ineffective by residual stresses believed to arise from the ultra-high pressure process. Particularly low fracture toughnesses of 2.0 to 3.4 MPam1/2 were determined for these composites and the Vickers hardness values were close to 20 GPa. Additional studies were made on these composites to examine a polytypic phase transformation (to 3C SiC) that was induced in the platelets by the extreme fabrication conditions. This was attributed to the co-operative motion of partial dislocations on the basal planes. The cBN-Si3N4 composites exhibited considerable plastic deformation of the cBN grains, in common with other superabrasive compacts. High Vickers hardness values above 35 GPa were achieved in these composites. However, the Knoop hardness values were lower than expected and the composites were found to have low strength and moderate fracture toughness, which was attributed to crack deflection and microcracking processes. The best fracture toughness value achieved was 5.81 ± 0.25 MPamI/2, obtained for a 60 wt% cBN composition which also exhibited the highest strength, 316 ± 12 MPa.